Ratio controller with zero ratio capability

ABSTRACT

A ratio controller having circuitry for establishing a zero ratio. A master signal provided by a master controller is applied to the upper end of a bridge circuit consisting of two parallel branches. The first branch includes a series-connected potentiometer winding. The second branch includes two seriesconnected resistors. The ratio of impedances in the second branch can be made equal to the ratio of impedances in the first branch taken between the nominal zero setting of the potentiometer and the upper and lower ends, respectively, of the bridge circuit. An amplifier has a first input connected to the potentiometer tap and a second input connected to the junction of the resistors in the second branch. The output of this amplifier represents the setpoint at which the local process variable should be maintained.

United States Patent Riley 211 Appl. No.: 93,156

[52] US. Cl. ..323/75 N, 137/88 [51] Int. Cl ..G01r 17/14 [58] FieldofSearch ..137/87, 88; 235/151.34;

[56] References Cited UNITED STATES PATENTS 2,891,218 6/1959 Werts..323/75 N 2,813,235 11/1957 Clay.... ..323/75 N 3,247,703 4/1966 Burk..323/75 N 3,378,024 4/1968 Kruik ..323/75 N 68 mi L VOL TAGE SOURCEPROCESS VARIABLE 7 [451 July 18, 1972 Primary Examiner-A. D. PellinenAItorney-William S. Wolfe, Frank L. Neuhauser, Oscar B. Waddell andJoseph B. Forman 1 1 ABSTRACT A ratio controller having circuitry forestablishing a zero ratio.

A master signal provided by a master controller is applied to the upperend of a bridge circuit consisting of two parallel branches. The firstbranch includes a series-connected potentiometer winding. The secondbranch includes two series-connected resistors. The ratio of impedancesin the second branch can be made equal to the ratio of impedances in thefirst branch taken betweenthe nominal zero setting of the potentiometerand the upper and lower ends, respectively, of the bridge circuit. Anamplifier has a first input connected to the potentiometer tap and asecond input connected to the junction of the resistors in the secondbranch. The output of this amplifier represents the setpoint at whichthe local process variable should be maintained.

8 Claim, 2 Drawing Figures PATENTED JUL 1 8 I972 TRANSDUCER TRANSDUCERLOCAL PROCESS VARIABLE VOLTAGE SOURCE n e m 3 F Iv N m l M m L R L A E AG C W O U 2 R W 4 D E M S R C R O N O U O 3 A o 1 D L o .l 4 S A 0/ R o3 N G O T 3 A 6 5 R l T R R R m u w 2 n mm mmm NU NL R L o a O .Q C C C2 4 3 6 S S 4 S 4 V V V O vo\ o vo I 2 4 R E u M E P V N G 8 m F MO 2 C2 MASTER 0 78 INVENTORZ JOHN E RILEY,

BY em: 3%

. ms ATTORNEY.

BACKGROUND OF THE INVENTION The present invention relates to processcontrollers and,

more particularly, to a ratio controller having the capability ofestablishing a local setpoint at any desired ratio, including a zeroratio, with respect to a master signal.

A common type of industrial process control system includes processcontrollers which regulate the flow of a mixture of raw materials whileconcurrently maintaining the composition of the mixture according to apredetermined formula. The control system includes a master controllerwhich responds to a process variable signal representing the flow rateof the mixture by generating a master signal representing the total flowcorrection required to maintain the mixture flow rate of a setpoint.Each of a plurality of ratio controllers connected to the mastercontroller modifies the master signal to establish a proportional localsetpoint signal. Each ratio controller controls the flow of one of theconstituents of the mixture as a function of the local setpoint and alocal process variable signal representing the actual flow rate of theconstituent. By altering the flow rate of each constituent, the mixtureflow rate is maintained at the setpoint.

In prior art ratio controllers, a local setpoint is established throughthe use of a potentiometer having its winding connected in series withthe output of the master controller. With the master signal appliedacross the potentiometer winding,

the local setpoint is established by adjusting the potentiometer tap atany position between the 100 percent setting and the nominal zerosetting. In a typical prior art controller, the minimum percentageattainable is not actually zero but is on the order of 5 percent due towinding resistance taken between the nominal zero setting of thepotentiometer tap and the adjacent winding terminal.

ln some instances, it may be desirable to maintain the ratio of aparticular local process variable at a ratio less than that attainablein prior art controllers. Also, the fact that the nominal zero settingof the ratio-controlling potentiometer is not an actual zero is thesource of calibration problems. By using different transducers togenerate process variable signals, identical process controllers can beused to regulate processes operating in different value ranges. Althoughinternal controller circuitry is independent of the value of a variablebeing monitored, an indicator associated with the process controller isnaturally marked according to the range of values through which theprocess variable is expected to vary. The manufacturer of processcontrol instruments must mark each indicator to show both the scalevalue of the process variable for a 1.0 ratio and also the actual valueof the process variable when the ratio is nominally zero. To simplifycalibration problems, it is desirable that a ratio controller have thecapability of generating a local setpoint at any predeterminedpercentage between an actual zero and 100 percent of a master signal.When the controller has this capability, the manufacturer can mark themaximum scale value only as the nominal zero corresponds to an actualzero which would preferably be pre-printed on the indicator scale.

SUMMARY OF THE lNVENTlON The present invention is a circuit which isused in a ratio controllertdestablish a local setpoint at any valuebetween 0 percent and 100 percent of the master signal. The circuitincludes a bridge circuit having a first branch including aseriesconnected potentiometer winding and a second parallel branch withfirst and second serially connected resistors. The master signal isapplied to the upper end of the bridge circuit. An amplifier has a firstinput connected to the potentiometer tap and a second relativelyinverted input connected to the junction of the first and secondresistors. The output of the amplifier is the local setpoint signal.When the ratio of the first and second impedances in-the second branchis equal to the ratio of impedances in the first branch taken betweenthe potentiometer tap at its nominal zero setting and the upper andlower ends, respectively, of the first branch, an actual zero ratioexists.

DESCRIPTION OF THE DRAWING While the specification concludes with claimsparticularly pointing out and distinctly claiming that which is regardedas the present invention, certain details of a preferred embodiment ofthe invention may be more readily ascertained from the followingdetailed description when read in conjunction with the accompanyingdrawings in which:

FlG. l is a block diagram of a typical process control system usingratio controllers; and

FIG. 2 is a partial schematic diagram ofa circuit capable ofestablishing a zero in a ratio controller.

DETAlLED DESCRlPTlON Referring now to FIG. 1, the process control systemshown in block diagram form controls the flow of a fluid mixture througha pipeline [0 connected to a fluid mixing tank 12 by controlling theflow of constituents entering the mixing tank 12 through pipelines 14,16 and 18. The flow of the mixture through pipeline 10 is monitored by asuitable flow transducer 20 which generates a process variable signalproportional to the flow rate. The process variable signal is comparedto a setpoint signal, SPM, in a master controller 22 wherein acorrective or master signal is generated. The master signal is appliedto a plurality of ratio controllers 24, 26, 28, each of which isassociated with a different one of the pipelines l4, l6, 18.

Each of the ratio controllers 24, 26, 28 contains means for scaling themaster signal provided by master controller 22 to provide a localsetpoint at a desired percentage of the master signal. For example, ifthe constituent flowing through pipeline 14 is to be 50 percent of thetotal flow through pipeline 10, circuitry in ratio controller 24establishes a local setpoint equal to 50 percent of the master signal. Aflow transducer 30 monitors the fluid flow through pipeline l4 andprovides a local process variable signal to the ratio controller 24. Thelocal process variable signal is compared to the local set point toobtain necessary corrective signals for adjusting the setting of a valve32 to correct the fluid flow through pipeline 14 by 50 percent of thetotal correction required in pipeline 10. The fluid flow throughpipeline 16 is similarly regulated in a closed control loop includingflow transducer 34, ratio controller 26 and valve 36. The control loopfor pipeline 18 consists of flow transducer 38, ratio controller 28 andvalve 40.

In one embodiment of the invention, the ratio controllers 24, 26, 28 maybe connected either to the master controller 22 or to conventionalmanually adjusted voltage sources V51, V82, VS3 through input switchingmeans 42, 44 and 46. In this embodiment of the invention, each of theratio controllers may be operated as a conventional process controllerindependently of the master controller 22.

The circuitry required to scale the master signal to obtain a localsetpoint signal is shown in greater detail in FIG. 2. The master signalis applied to the non-inverting input of a high input impedanceamplifier 48 having an output terminal connected to the upper end of abridge circuit 50 with first and second parallel-connected branches 52and 54. The first branch 52 includes a first adjustable resistor 56, thewinding 57 of a potentiometer 58 and a second adjustable resistor 60.The adjustable resistor 56 is used to establish the range through whicha ratio may be varied while the potentiometer 58 is used to establishthe precise ratio within a pre-selected range. The second branch 54 ofbridge circuit 50 includes a first resistor 62 and a second resistor 64.

A switching circuit consisting of an upper contact set 66a and a lowercontact set 66b may be included in branch 52 to permit the potentiometer58 to be energized either by the master controller output or by amanually adjustable voltage source 68. The source 68 provides a constantsetpoint signal independent of the process conditions.

In one embodiment of the invention, the output of amplifier 48 variesthrough a l to 5 voltage range. The 1 volt is eliminated from the signalimposed across the bridge circuit 50 by means of another high inputimpedance follower amplifier 70 having a fixed bias voltage applied toits non-inverting input terminal. By adjusting the input voltage to theamplifier 70, the output is adjusted to 1.0 volt so that the voltageacross the bridge circuit varies between 0 and 4.0 volts.

The tap 59 of potentiometer 58 is connected to the non-inverting inputterminal of an amplifier 72 while the junction of the resistors 62 and64 in the second branch 54 is connected to the inverting input terminalof amplifier 72 through a switch 74. When the ratio controller is to beused as a conventional controller with a manually adjusted setpoint,switch 74 disconnects the inverting input of amplifier 72 from thesecond branch of the bridge circuit 50. Under these conditions, thevoltage applied to the non-inverting input terminal of the amplifier 72is determined entirely by the voltage applied across potentiometer 58 byvoltage source 68.

Under normal conditions, however, the voltage source 68 is opencircuited and the switch 74 connects the junction of the resistor 62 and64 to the inverting input terminal of amplifier 72. The output of theamplifier 72 represents a local setpoint that is applied to theinverting input terminal of a differential amplifier 76 where it iscompared to a local process variable signal applied to the non-invertinginput terminal of the amplifier 76. In a preferred embodiment oftheinvention, the local process variable signal is generated by a localtransducer (not shown) and is applied to a high input impedance followeramplifier 78 before being applied to the non-inverting input terminal ofthe differential amplifier 76, It should be understood that the localsetpoint signal and the local process variable signal could be appliedto opposite input terminals of differential amplifier 76 if required inthe control of a particular process.

In the circuit described above, an absolute zero ratio may be achievedas follows. Once the potentiometer tap 59 is located in its nominal zerosetting, the resistor 60 is adjusted until the ratio of resistance inthe first and second branches is equal. More particularly, the ratio ofresistor 62 to resistor 64 is made equal to the ratio of the resistancein the first branch taken between the upper end of the bridge circuitand tap 59 and between the tap 59 and the lower end of the bridgecircuit. When these impedance ratios are equalized, the bridge isbalanced and the same voltage is applicable to both amplifier inputs.Changes in the signal applied to bridge circuit 50 will result in equalvoltage changes at both inputs to amplifier 72.

While there has been described what is presently thought to be apreferred embodiment of a ratio controller having zero ratio capability,it should be understood that variations and modifications therein willoccur to those skilled in the art once they become familiar with theinvention. Therefore, it is intended that the appended claims shall beconstrued to include all such variations and modifications as fallwithin the true spirit and scope of the invention.

lclaim:

1. In a process control system having a master controller and at leastone local controller, each of which is to maintain a local processvariable at a local setpoint, a circuit for establishing a localsetpoint'as a predetermined percentage, between 0 and 100, of a mastersignal comprising:

a. a bridge circuit having a first leg with a potentiometer and a secondparallel leg with first and second serially connected resistors;

b. means for applying the master signal to one junction of said firstand second legs;

c. an amplifier having a first input connected to the tap on saidpotentiometer and a second complementary input connected to the junctionof said first and second resistors, the output of said amplifier beingthe local setpoint; and

d. means for adjusting impedances in said bridge circuit to establishthe same ratio of imfpedances in each leg taken between said onejunction 0 said first and second legs and each amplifier input andbetween each amplifier input and the other junction of said first andsecond legs when the potentiometer tap is at its nominal zero setting.

2. A circuit as recited in claim 1 wherein said adjusting meanscomprises an adjustable third resistor connected in the first legbetween the potentiometer and said other junction of said first andsecond legs.

3. A circuit as recited in claim 2 further including a spancontrollingadjustable fourth resistor connected in said first leg between thepotentiometer and said one junction of said first and second legs.

4. A circuit as recited in claim 3 in further combination with:

a. means for generating a local process variable signal;

b. a differential amplifier having a first input terminal connected tothe output of said first-mentioned amplifier, a second input terminalconnected to the output of said local process variable signal generatingmeans, and an output terminal; and

c. a control element connected to the output terminal of saiddifferential amplifier for altering the process to drive the localprocess variable toward the established local setpoint.

5. A circuit as recited in claim 2 in further combination with:

a. means for generating a local process variable signal;

b. a differential amplifier having a first input terminal connected tothe output of said first-mentioned amplifier, a second input terminalconnected to the output of said local process variable signal generatingmeans, and an output terminal; and

c. a control element connected to the output terminal of saiddifferential amplifier for altering the process to drive the localprocess variable toward the established local setpoint.

6. A circuit as recited in claim 1 further including a spancontrollingadjustable fourth resistor connected in said first leg between thepotentiometer and said one junction of said first and second legs.

7. A circuit as recited in claim 6 in further combination with:

a. means for generating a local process variable signal;

b. a differential amplifier having a first input terminal connected tothe output of said first-mentioned amplifier, a second input terminalconnected to the output of said local process variable signal generatingmeans, and an output terminal; and

c. a control element connected to the output terminal of saiddifferential amplifier for altering the process to drive the localprocess variable toward the established local setpoint.

8. A circuit as recited in claim 1 in further combination with:

a. means for generating a local process variable signal;

b. a differential amplifier having a first input terminal connected tothe output of said first-mentioned amplifier, a second input terminalconnected to the output of said local process variable signal generatingmeans, and an output terminal; and

c. a control element connected to the output terminal of saiddifferential amplifier for altering the process to drive the localprocess variable toward the established local setpoint.

1. In a process control system having a master controller and at leastone local controller, each of which is to maintain a local processvariable at a local setpoint, a circuit for establishing a localsetpoint as a predetermined percentage, between 0 and 100, of a mastersignal comprising: a. a bridge circuit having a first leg with apotentiometer and a second parallel leg with first and second seriallyconnected resistors; b. means for applying the master signal to onejunction of said first and second legs; c. an amplifier having a firstinput connected to the tap on said potentiometer and a secondcomplementary input connected to the junction of said first and secondresistors, the output of said amplifier being the local setpoint; and d.means for adjusting impedances in said bridge circuit to establish thesame ratio of impedances in each leg taken between said one junction ofsaid first and second legs and each amplifier input and between eachamplifier input and the other junction of said first and second legswhen the potentiometer tap is at its nominal zero setting.
 2. A circuitas recited in claim 1 wherein said adjusting means comprises anadjustable third resistor connected in the first leg between thepotentiometer and said other junction of said first and second legs. 3.A circuit as recited in claim 2 further including a span-controllingadjustable fourth resistor connected in said first leg between thepotentiometer and said one junction of said first and second legs.
 4. Acircuit as recited in claim 3 in further combination with: a. means forgenerating a local process variable signal; b. a differential amplifierhaving a first input terminal connected to the output of saidfirst-mentioned amplifier, a second input terminal connected to theoutput of said local process variable signal generating means, and anoutput terminal; and c. a control element connected to the outputterminal of said differential amplifier for altering the process todrive the local process variable toward the established local setpoint.5. A circuit as recited in claim 2 in further combination with: a. meansfor generating a local process variable signal; b. a differentialamplifier having a first input terminal connected to the output of saidfirst-mentioned amplifier, a second input terminal connected to theoutput of said local process variable signal generating means, and anoutput terminal; and c. a control element connected to the outputterminal of said differential amplifier for altering the process todrive the local process variable toward the established local setpoint.6. A circuit as recited in claim 1 further including a span-controllingadjustable fourth resistor connected in said first leg between thepotentiometer and said one junction of said first and second legs.
 7. Acircuit as recited in claim 6 in further combination with: a. means forgenerating a local process variable signal; b. a differential Amplifierhaving a first input terminal connected to the output of saidfirst-mentioned amplifier, a second input terminal connected to theoutput of said local process variable signal generating means, and anoutput terminal; and c. a control element connected to the outputterminal of said differential amplifier for altering the process todrive the local process variable toward the established local setpoint.8. A circuit as recited in claim 1 in further combination with: a. meansfor generating a local process variable signal; b. a differentialamplifier having a first input terminal connected to the output of saidfirst-mentioned amplifier, a second input terminal connected to theoutput of said local process variable signal generating means, and anoutput terminal; and c. a control element connected to the outputterminal of said differential amplifier for altering the process todrive the local process variable toward the established local setpoint.